Three-Dimensional Hydrodynamics and Morpho-dynamics at a Tidal River Junction

Abstract

River junctions act as critical nodes in river networks because they can affect flows, sediment transports, and morphological and ecological patterns. River junctions subject to the unidirectional flow have been widely studied in the last decades; in contrast, the efforts are limited regarding the understanding of flow behaviors and morphological changes around tidal river junctions. In this study, a numerical model coupling two- and three-dimensional (2D-3D) domains is established to study the flow patterns and sediment motion of the tidal reach of Rongjiang River (RR), which has a typical tidal river junction. The simulation results show that the continuous process of alternating merging and separating streams leads to the swing of flow dynamic axes in the planar field, and results in the periodic inversion of secondary flows (helical flows) around the junction. These features are unique and different from the hydrodynamics of fluvial junctions. Moreover, a simulation of the particle moving indicates that the periodic 3D circulation around the junction can make the suspended sediment tend to gather in the north branch, which leads to the net input of the sediment into the north branch. Additionally, the long-term morphological evolutions and potential changes are analyzed by historical data and a numerical experiment. The numerical experiment results illustrate the significant sedimentation at river banks and deepening at mid-channels under the effects of tidal currents, which also demonstrates that the net sediment input to the tributary is a potential cause and mechanism of the distinct bed discordance in tidal river junctions. Furthermore, these findings emphasize the importance of periodical flow behaviors and local hydraulics on the dynamics around the tidal river junctions, which can expand the understanding of physics both in tidal and non-tidal river reaches and provide a reference for tidal river management.

Appendices

Appendix 1 Harmonic Analysis Results of the Tidal Level at Mayu

Tidal harmonic analysis, a least-squares fitting procedure, is normally used for the determination of tidal constituent phases and amplitudes in tidal systems. In this study, we take notice of the tidal characteristics at the RR mouth (Mayu Station). The long-term tidal level data of Mayu (from 1954 to 2017, except 1992) are extracted from the hydrological records, and all water level records are converted to Pearl River Datum. The tidal levels are analyzed using the T_TIDE program (Pawlowicz et al. 2002) as a sum of a sinusoidal wave with different frequencies. By the harmonic analysis here, the mean tidal level, the partial tide amplitude, and phases can be obtained.

Tidal properties are characterized by performing the harmonic analysis of the dominant constituents: O₁, K₁, M₂, and S₂. As for the other tide constituents, their amplitudes are all less than 0.05 m. Figure 17 shows the harmonic analysis results of the tidal levels at Mayu. Firstly, the annual mean water level at Mayu has been on the rise overall. Among the main constituents, M₂ is the most significant and has the largest amplitudes. Moreover, for O₁ and K₁, both their amplitudes and phases change periodically, which also corresponds to the 18.61-year cycle for tides. For S₂, it has no obvious annual variation of amplitudes and phases.

Fig. 17

The annual mean values of tidal levels (a), amplitudes (b), and phases (c) at Mayu. The green line in b represents the annual mean values of the form factor (FF)

To assess the classification of tides, a form factor (FF) is used, and it is defined (Rose and Bhaskaran 2017; Dai et al. 2019):

$$FF=\frac{{A}_{{O}_{1}}+{A}_{{K}_{1}}}{{A}_{{M}_{2}}+{A}_{{S}_{2}}}$$

where A_(*) represent amplitude of the tidal constituent (*). The ranges of the FF for different classifications of tides are shown in Table 2. The results illustrate that tides at Mayu are of the mixed and mainly semidiurnal type.

Table 2 Different classifications of tides and value ranges of the form factor (FF)

Additionally, the duration of tidal flood and ebb is analyzed. The annual average durations of tidal rise and fall are 6.97 h and 5.45 h, respectively. That is to say, the duration of the flood is larger than that of the ebb in the RR estuary, which indicates that there is an ebb tidal asymmetry here.

Appendix 2 Model Validation Results

The model validation results are shown in Fig. 18.

Fig. 18

Validation results of the a water level, b surface velocity, and c sediment concentration. The positive velocity coincides with the seaward flow (tide falling). The hollow dots represent the measured data, and the lines represent the simulation results